Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Oxygenates such as alcohols, particularly methanol, dimethyl ether, and ethanol, are alternative feedstocks for the production of light olefins. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
In an oxygenate to olefin (OTO) reaction system, an oxygenate in an oxygenate-containing feedstock contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenate to light olefins, which are yielded from the reaction system in a reaction effluent. As the feedstock contacts the molecular sieve catalyst compositions at high weight hourly space velocities and under extreme temperature and pressure conditions, a portion of the catalyst compositions can shear or break away, e.g., attrite, to form one or more smaller catalyst attrition particles. Some catalyst attrition particles are very small in size and are referred to as catalyst fines. Due to their relatively high surface area to mass ratios, a portion of the catalyst fines in the reaction system may become undesirably entrained with the reaction effluent and exit the reaction system therewith. Conversely, due to their relatively low surface area to mass ratios, larger particles tend to be selectively retained in OTO reaction systems. The selective retention of larger particles is particularly a problem for highly attrition resistant particles.
The build up of large catalyst particles in an OTO reaction system produces two undesirable effects. First, in a large particle rich reaction system, the circulating fluid bed will not operate as well, particularly with regard to circulation of catalyst within the standpipes. Second, the large particles that are selectively retained in the reaction system will tend to lose their effectiveness, e.g., activity and selectivity, with time. That is, the build up of large particles in the reaction system is undesirable because the larger particles will tend to decrease the overall effectiveness of the collection of catalyst particles contained in the reaction system.
One conventional technique for removing undesirably-sized catalyst particles includes non-selectively removing a fraction of all of the catalyst particles in the reaction system to make room for the addition of fresh catalyst. This technique for removing undesirably-sized catalyst particles is inefficient, however, because a significant portion of the desirably-sized catalyst particles are removed from the reaction system with the undesirably-sized catalyst particles.
U.S. Pat. No. 5,746,321 to Hettinger, Jr. et al. discloses the combination of a magnetic separator, a catalyst classifier, and/or a catalyst attriter, which wears off the outer layers of catalyst, yields more active catalyst of lower metal content with closer control of average particles size, and narrows particle size distribution providing improved fluidization properties and better activity and selectivity. The process is especially useful when processing high metal-containing feedstocks.
U.S. Pat. No. 2,573,559 to Friedman discloses replacing a bed of fluidized catalyst, which has become reduced in activity during use, with fresh fluidized catalyst, the average particle size of both catalysts being in the range of 40–400 mesh. The average size of the fresh catalyst differs from that of the partially spent catalyst by at least 10-mesh size and preferably 25 mesh. The fresh catalyst is introduced into the reactor under conditions such that the reaction temperature is not substantially increased, and at the same time catalyst is withdrawn from the reactor at a portion below the top level of the bed. The catalyst withdrawn is separated by particle size into fresh catalyst, which is returned to the reactor, and deactivated catalyst, which is regenerated. According to the '559 patent, the complete replacement of catalyst can be accomplished under normal operating conditions in from 20 to 48 hours.
In certain reaction systems, the removal of small catalyst particles may be desired. For example, if a reaction system implements catalyst compositions having low attrition resistance, then the catalyst compositions may, depending on reaction conditions, readily attrite and form catalyst fines. The increase in the proportion of catalyst fines in the reaction system will decrease the median particle diameter of the entire catalyst population in the reaction system, which may undesirably impact the fluidization characteristics within the reaction system. Undesirably high levels of fines may make it difficult to maintain desired fluid bed densities and thus adversely affect the reaction rates associated with the reaction zone. As a result, it may be desired to selectively remove small catalyst particles, e.g., catalyst fines, from a reaction system in order to maintain a desired particle size distribution in the reaction system.
In view of the importance of maintaining desirably-sized catalyst particles in reaction systems, particularly in OTO reaction systems, improved processes are sought for selectively removing undesirably-sized catalyst particles from OTO reaction systems. More specifically, improved processes are sought that can maintain a desirable catalyst particle size distribution in an OTO reaction system and thereby provide desirable fluidization and catalytic activity characteristics.